shower and toilet cause pipe rattle

[strainsa]

I live in a 9 year old house. It has a secondary water heater in the attic servicing 2 bathrooms
(3 sinks, 2 toilets, 1 shower, 1 tub). When the shower is turned on first thing in the morning and one of
the two toilets is flushed, when the toilet completes its fillup (shuts off), the pipes in the wall rattle
until a sink or the shower water is cycled (on/off). I have taken out the shower faucet cartridge
assembly (single handle delta unit) and it rattles when shaken. The toilets have replacement works
in the tank (not the older hissing shutoff valves). Any idea what might be causing this or what to look for?

[X Y Scaling]

Sounds like a water hammer to me...... If you are still around, I'll go into details.....

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